1. Field
The present application relates to a method for fabricating a liquid crystal display (LCD) device and, more particularly, to a method for fabricating an LCD device capable of enhancing productivity by improving a masking process with a diffraction mask, and the diffraction mask used therefor.
2. Description of the Related Art
Increasing concentration on information display and demands on a mobile information medium have created a drive for research of a light-weight flat panel display (FPD) to replace existing display device CRT (Cathode Ray Tube) technology. In particular, among various types of flat panel displays, liquid crystal displays (LCD) are predominantly used in notebook or desktop computers thanks to their excellent resolution, color display and picture quality.
In general, the LCD displays a desired image by individually supplying a data signal according to image information to liquid crystal cells arranged in a matrix form and controlling light transmittance of the liquid crystal cells.
For this purpose, the LCD includes a liquid crystal display panel including a driving circuit unit and outputting an image, a backlight unit installed at a lower portion of the liquid crystal display panel and emitting light to the liquid crystal display panel, and a case for coupling the backlight unit and the liquid crystal display panel.
The liquid crystal display panel includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
The color filter substrate includes a color filter consisting of red, green and blue sub-color filters implementing color, a black matrix dividing the sub-color filters and blocking a light from transmitting a liquid crystal layer, and a transparent common electrode applying a voltage to the liquid crystal layer.
The array substrate includes a plurality of gate lines and data lines arranged vertically and horizontally on the substrate to define a plurality of pixel regions, a thin film transistor (TFT), a switching device, formed at the crossings of the gate lines and the data lines, and a pixel electrode formed on the pixel region.
The color filter substrate and the array substrate are attached by a sealant formed at an outer edge of an image display region to constitute a liquid crystal display panel. Attachment of the two substrates is made through an attachment key formed at the color filter substrate or the array substrate.
A thin film transistor is used as the switching device of the liquid crystal display panel. An amorphous silicon thin film or a polycrystalline silicon thin film is used as a channel layer of the thin film transistor.
The polycrystalline silicon thin film transistor has high field effect mobility, so that an operation frequency of a driving circuit unit determining the number of driving pixels can be enhanced and thus the display device can have a fine pitch. In addition, since a charge time of a signal voltage of a pixel unit is reduced, distortion of a transmission signal can be reduced, and thus, picture quality can be improved.
Moreover, because the polycrystalline silicon TFT can be driven with a voltage less than 10V, compared to the amorphous silicon TFT having a high driving voltage (˜25V), power consumption can be reduced.
As mentioned above, the LCD is fabricated using several masking processes (namely, photolithography processes), which will be described as follows.
FIGS. 1A to 1G are sequential sectional view of a general process of fabricating an LCD, showing an array substrate of a trans-reflective LCD with a pixel electrode consisting of transmission electrode and reflection electrode.
As shown in FIG. 1A, a polycrystalline silicon TFT is formed on a transparent substrate 10 and patterned through a first masking process to form an island type active pattern 11.
Next, as shown in FIG. 1B, a first insulation film 13 and a first metal film are sequentially deposited at the entire surface of the substrate 10 with the active pattern 11 formed thereon, and then, the first metal film is patterned through a second masking process to form a gate electrode 15 with the first insulation film 13 formed on the active pattern 11.
And then, impurity ions are injected at both sides of the active layer 11 by using the gate electrode 15 as a mask to form source/drain regions 11a and 11b. 
Thereafter, as shown in FIG. 1C, a second insulation film 14 is deposited at the entire surface of the substrate 10 with the gate electrode 15 formed thereon, and then a portion of the first and second insulation films 13 and 14 are removed through a third masking process, thereby forming first and second contact holes 17b exposing a portion of the source and drain regions 11a and 11b. 
And then, as shown in FIG. 1D, a transparent conductive material is deposited on the second insulation film 14 with the first and second contact holes 17a and 17b formed and patterned through a fourth masking process to form a pixel electrode 21.
Subsequently, as shown in FIG. 1E, a second metal film is formed at the contact holes 17a and 17b and the pixel electrode 21 and patterned through a fifth masking process to form source and drain electrodes 19a and 19b. 
The source electrode 17a is electrically connected with the source region 11a through the first contact hole 17a and the drain electrode 17b is electrically connected with the drain region 11b through the second contact hole 11b. 
And then, as shown in FIG. 1F, a third insulation film 16 is deposited at the entire surface of the substrate 10 including the source/drain electrodes 19a and 19b and patterned through a sixth masking process to form a third contact hole 17b exposing a portion of the drain electrode 19b, and expose a pixel electrode of the transmitting part (T).
Finally, as shown in FIG. 1G, a third metal film is deposited on the third insulation film 16 including the third contact hole 17c and patterned through a seventh masking process to form a reflection electrode 23 exposing the transmitting part (T). In this case, the reflection electrode 23 is electrically connected with the drain electrode 19b through the third contact hole 17c, the region of which becomes a reflecting part (R).
As mentioned above, the lower array substrate of the general trans-reflective LCD is fabricated through seven masking processes, namely, photolithography processes. However, each photolithography process is a series of processes for forming a desired pattern by transferring a pattern drawn on a mask to the thin film-deposited substrate by coating sensitizing solution on the substrate, exposing and developing the solution. As a result, the photolithography processes degrade production yield and heighten the probability that the TFT is defective.
In addition, since the mask designed for forming the pattern is very expensive, an increase in the number of masks applied to the processes leads to a proportional increase in the fabrication cost of the LCD.